1. Field of the Invention
The present invention relates to a method of forming a polarization inversion portion by applying electric field to a ferroelectric crystal, more particularly to a polarization inversion method of ferroelectrics which is capable of forming a uniform polarization inversion pattern by shortening an electric field application time. Furthermore, the present invention relates to a method of fabricating an optical wavelength conversion device, which applies the polarization inversion method of the ferroelectric.
2. Description of the Related Art
A method has been already proposed by Bleombergen et al., which changes a wavelength of a fundamental wave to that of a second harmonic by use of an optical wavelength conversion device providing an area in which a self-polarization (domain) of ferroelectric showing a nonlinear optical effect is periodically inverted (see Physical Review, Vol. 127, No. 6, pp. 1918 (1962)). In this method, a cycle xcex9 of a polarization inversion portion is set to an integer multiple of a coherence lengthxcex9c which is given by the following equation (1)
xcex9c=2xcfx80/{xcex2(2xcfx89)xe2x88x922xcex2(xcfx89)}xe2x80x83xe2x80x83(1) 
where xcex2 (2xcfx89) is a propagation constant of a second harmonic and xcex2 (xcfx89) is a propagation constant of a fundamental wave. Thus, a phase matching of the second harmonic can be achieved. When the wavelength conversion is performed by use of a bulk crystal of a nonlinear optical material, a wavelength that is subjected to the phase matching is limited to a specific wavelength which is inherent to the crystal. However, according to the above described method, the phase matching that is a so-called pseudo phase matching can be effectively achieved by using the cycle xcex9 satisfying the equation (1) for an desired wavelength.
As a method forming the above described periodic polarization inversion structure, one has been known as described in Japanese Unexamined Patent Publication No.7(1995)-72521. In this method, after periodic electrodes having a predetermined pattern are formed on one surface of a ferroelectric crystal showing a nonlinear optical effect, which has been subjected to a single polarization, a ferroelectric crystal is corona-charged by the periodic electrodes and a corona wire disposed on a surface opposite to that one surface, and electric field is applied to the ferroelectric crystal, thus converting a portion opposite to one of the periodic electrodes of the ferroelectric crystal to a local polarization inversion portion.
In addition to the method which applies the corona charging, a method has been known as is described in Japanese Patent No. 3005225, in which an entire surface electrode is formed on the entire surface of ferroelectric opposite to a surface where periodic electrodes having a predetermined pattern are formed, and electric field is directly applied to the ferroelectric by use of the entire surface electrode and the periodic electrodes, thus forming a local polarization inversion portion.
When the polarization of the ferroelectric crystal is inverted by any of the above described conventional methods, one polarization inversion portion is formed in the portion opposite to a corresponding one of the periodic electrodes of the ferroelectric crystal.
Incidentally, in the case where a periodic polarization inversion structure having a particularly long cycle or a large area is formed in forming the periodic polarization inversion portions in the ferroelectric crystal by use of the above described electrodes, a long electric field application time is required. If the electric field application time is long, a problem is recognized, in which an inversion width is large in an earlier polarization inversion portion where a polarization inversion first starts, and the inversion width is small in a latter polarization inversion portion where the polarization inversion is started later, thus creating unevenness of the width of the polarization inversion portion.
When the foregoing problem occurs in the optical wavelength conversion device in which the periodic polarization inversion structure is formed in a ferroelectric crystal showing then on linear optical effect, a cycle or width ratio of the polarization inversion portions is uneven and a decrease in wavelength conversion efficiency is brought about.
In consideration of the foregoing circumstances, an object of the present invention is to provide a polarization inversion method of ferroelectrics, which is capable of accurately forming polarization inversion portions having a desired pattern in a short electric field application time.
Another object of the present invention is to provide a method of fabricating an optical wavelength conversion device, which is capable of forming a periodic polarization inversion structure having an excellent periodicity in a nonlinear optical crystal that is ferroelectric.
A first polarization inversion method of ferroelectric of the present invention is one in which one electrode having a shape corresponding to the area is not disposed in an area of a ferroelectric crystal where a polarization is desired to be inverted, but one in which a plurality of electrodes smaller than the area are disposed in this area, and electric field is applied to the ferroelectric crystal through the plurality of electrodes, the method comprising the steps of: forming electrodes having a predetermined pattern on one surface of the ferroelectric crystal that has been subjected to a single polarization; and forming a local polarization inversion portion in the ferroelectric crystal by applying electric fields to front and back surfaces of the ferroelectric crystal via the electrodes, wherein one polarization inversion portion having a desired pattern is formed by allowing portions of the ferroelectric crystal respectively corresponding to the plurality of electrodes and portions between these portions to be polarization-inverted.
A second polarization inversion method of ferroelectrics of the present invention is a method in which the first method is applied especially to a case where a periodic polarization inversion structure is formed. In this second method, periodic electrodes are used as the foregoing electrodes, which include a plurality of electrode groups formed periodically, each being composed of a plurality of electrodes; and each polarization inversion portion is formed for a corresponding one of the electrode groups, thus forming a periodic polarization inversion structure in which the polarization inversion portion is periodically and repetitively formed.
In the polarization inversion method of ferroelectric according to the present invention, a corona wire is disposed on one surface of a ferroelectric crystal opposite to one surface thereof, and electric field should be applied by corona charging by use of the corona wire and the electrodes.
The first and second polarization inversion methods of ferroelectric of the present invention are particularly effective when the ferroelectric crystal is a LiNbxTa1-xO3 (0 less than x less than 1) crystal or a crystal doped with one of MgO, ZnO and Sc.
On the other hand, a method of fabricating an optical wavelength conversion device of the present invention is a method to which the foregoing second polarization inversion method of ferroelectrics of the present invention is applied. In this method of fabricating an optical wavelength conversion device of the present invention, a nonlinear optical crystal is used as a ferroelectric crystal that has been subjected to a single polarization, and a periodic polarization inversion structure corresponding to a periodic pattern of the foregoing one group of the electrodes is formed in the nonlinear optical crystal.
In general, when a polarization of the ferroelectric crystal is inverted, it is proved by experiments that an inversion nucleus first occurs, and then a polarization inversion grows around the inversion nucleus. When electric field is applied to the ferroelectric crystal via the electrode, inversion nuclei that are the portions illustrated by the oblique lines occur in the ends of the electrode 51 of the ferroelectric crystal 52 as shown in FIG. 6A, and the inversion nuclei grow as shown in FIG. 6B. Then, the inversion nuclei connect with each other, and finally the polarization inversion portion 53 having a shape corresponding to the electrode 51 is formed as shown in FIG. 6C.
When the periodic polarization inversion structure was formed by the conventional method, the periodic electrodes 51 were previously formed in the ferroelectric crystal 52 as shown in FIG. 7A, and electric field was applied to the ferroelectric crystal 52 via the electrodes 51. With such an electric field application, the polarization inversion portions 53 were allowed to be grown from the electrodes 51 to regions outside the electrodes 51, respectively, as shown in FIG. 7B, and the polarization inversion portions having a desired pattern were formed. Particularly, when a cycle of the polarization inversion portions is long, it has been required to make an electric field application time longer in forming a desired pattern with, for example, an aspect ratio of 1:1.
When the electric field application time, that is, an inversion time was long as in the conventional method, it was proved that charges are apt to concentrate at an earlier polarization inversion portion, that is, at a portion where the inversion nucleus first occurs and this causes unevenness of a width of the polarization inversion. This phenomenon was remarkably recognized in a LiNbxTa1-xO3 (0 less than X less than 1) doped with one of MgO, ZnO and Sc, in which an electrical conductivity of the ferroelectric crystal changes greatly at the boundary of the polarization inversion and charges are prone to concentrate in the earlier polarization inversion portion.
In the first polarization inversion method of ferroelectrics of the present invention, the plurality of electrodes smaller than areas of the ferroelectric crystal where the polarization is desired to be inverted are previously arranged therein, and the electric field is applied to the ferroelectric crystal via the plurality of electrodes. Accordingly, compared to the conventional method in which one electrode having the shape corresponding to the area where the polarization is desired to be inverted is formed therein, more electrode terminals exist in one area where the polarization is desired to be inverted.
In FIGS. 8A and 8B, shown is a state where the polarization inversion portions 53 with a desired pattern are formed in the ferroelectric crystal 53. In FIGS. 8A and 8B, reference numeral 51 denotes electrodes, and, in FIGS. 8A and 8B, a case in which one polarization inversion portion 53 is formed by the two electrodes 51 is exemplified.
As described above, if more electrode terminals exist where the inversion nuclei occur, the inversion nuclei occur with a high density, so that the polarization inversion portions can be formed in desired areas within a short electric field application time. Accordingly, a drawback caused by a low density of the occurrence of the inversion nuclei and unevenness of the inversion nuclei, that is, a drawback in which the polarization inversion area is broad in the earlier polarization inversion portion and the polarization inversion area is narrow in the latter polarization inversion portion where the polarization inversion starts later is avoided, and the polarization inversion portion having a desired pattern can be formed accurately.
In the first polarization inversion method of ferroelectric of the present invention, periodic electrodes in which the electrode groups composed of the plurality of electrodes are arranged periodically are used as the electrode for use in the electric field application, and one polarization inversion portion is formed for a corresponding one of the electrode groups, thus forming the periodic polarization inversion structure in which the polarization inversion portion is periodically arranged. Accordingly, the second polarization inversion method of ferroelectric of the present invention can avoid a drawback in which the width of the polarization inversion is broad in the earlier polarization inversion portion and the width of the polarization inversion is narrow in the latter polarization inversion portion, and a periodic polarization inversion structure having a uniform cycle and width ratio can be formed.
On the other hand, the method of fabricating an optical wavelength conversion device of the present invention is a method to which the foregoing second polarization inversion method of ferroelectrics is applied. Since the periodic polarization inversion structure corresponding to the periodic pattern of the electrode group is formed in the ferroelectric crystal that is a nonlinear optical crystal, the method of fabricating an optical wavelength conversion device of the present invention can fabricate an optical wavelength conversion device showing high wavelength conversion efficiency, which comprises the periodic polarization inversion structure having a uniform periodicity.
The method of fabricating an optical wavelength conversion device of the present invention is effective when this method is applied to a fabrication of an optical wavelength conversion device which performs a wavelength conversion for infrared-zone light. Specifically, since such a kind of optical wavelength conversion device shows a comparatively long cycle of a polarization inversion portion, that is, a comparatively broad width of the polarization inversion portion, a necessary electric field application time is long, so that a width of the polarization inversion is apt to be uneven. However, occurrence of such a drawback can be reliably prevented with an application of the present invention.
If the cycle of the polarization inversion portions is comparatively long as described above, distance between the electrodes is comparatively broad, and it is easy to form a divided electrode group. On the contrary, when the polarization inversion portions of a short cycle are formed, the width of each electrode in one divided electrode group need be made very small, and working of the electrode is very difficult.
As the number of the electrodes constituting one electrode group is larger, more electrode terminals exist where the inversion nuclei occur, and the foregoing effects of the present invention become conspicuous. However, the width of each electrode becomes narrower as the number of the electrodes is made larger, resulting in difficulty of the working of the electrode. Accordingly, the number of the electrodes constituting one electrode group should be set properly in consideration for both of the effects and the workability.